The present invention relates generally to an apparatus for generating optical radiation, and more particularly, to a diode pumped optical parametric oscillator involving three wave interaction to generate frequency tunable laser beams.
The desirability of providing a high quality, tunable laser is well known. Many commercial applications such as environmental sensing of air pollutants and other remote sensing applications such as searching for natural gas leaks, searching for gas and oil fields and spectroscopy in general would be greatly benefited by a high quality, tunable laser. As is known in the art, if atoms or molecules that absorb light at a specific wavelength are illuminated with light of that wavelength, they can be detected with an appropriate viewer. In this way, remote sensing for pollutants, etc. by a choice of illumination wavelength is enabled. As can be appreciated, the greater the extent to which a laser is tunable, the greater utility it will have in applications such as these.
Devices known as optical parametric oscillators are sometimes utilized in the above applications because they operate to convert a first or pump laser beam into two, lower frequency beams commonly known as signal and idler beams. The signal and idler beams have wavelengths longer than that of the pump beam.
Optical parametric oscillators utilize a nonlinear process in a medium to produce signal and idler beams. The wavelengths are determined by the physical requirements that momentum and energy be conserved. These two conservation laws result in the following equations for collinear phasematching:npumpωpump=nsignalωsignal+nidlerωidler  ω(pump)=ω(signal)+ω(idle) wherein ω represents frequency and n represents the refractive index, a measure of the speed of light in the nonlinear material. Since refractive index is a function of frequency, crystal orientation, and beam polarization, it is possible in some cases to simultaneously fulfill the requirements of both equations above.
A common characteristic of optical parametric oscillators is that they utilize one or more nonlinear crystals placed within a reflective cavity. The interaction of the pump beam with the nonlinear crystal gives rise to the generation of the signal and idler beams described in the equations above. The nonlinear crystal can be angularly manipulated with respect to the pump beam to provide a tuning effect; other effects such as changing the temperature of the nonlinear crystal can also be used for tuning.
A recently developed configuration for an optical parametric oscillator, described by Bosenberg. et al., Continuous-wave singly resonant optical parametric oscillator based on periodically poled LiNbO3, Optics Letters, Vol. 21, No. 10, May 15, 1996, Optical Society of America, includes a Periodically Poled Lithium Niobate (PPLN) crystal as the nonlinear medium. This device represents a major advance over other optical parametric oscillator embodiments. High nonlinear gain, no birefringent walkoff effects, and engineerable grating periodicity make cw optical parametric oscillators a practical reality for the first time. But this device uses a neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal pumped by diode lasers to provide the high power pump beam. While this device represents an advancement over the art, it is not without the need for improvement. More specifically, this device is complex because the diode laser light must be carefully coupled to the Nd laser. Moreover, Nd lasers are somewhat inefficient, typically converting only 30-40% of the diode pump power to Nd laser output power. The rest of the pump power becomes heat which must be dissipated.
While a direct substitution of the Nd:YAG laser with a high power diode laser would overcome the above described efficiency problem, as well as simplify the device, high power diode lasers typically suffer from an inherent poor beam quality, rendering them unsuitable for optical parametric oscillator applications.
A need exists therefore for an improved optical parametric oscillator pumped by an improved high efficiency laser source. Such a laser source would combine the desirable qualities of high power output, high beam quality for use within an optical parametric oscillator to provide high power, high quality output.